The availability of potassium in some Tasmanian soils. I. The variability of soil potassium in the field and its fractionation

Abstract

The potassium status of Frodsley sandy loam and some associated soils from the Fingal district, north-eastern Tasmania, has been investigated. Regularly spaced sampling of surface soils on a rectilinear grid pattern disclosed a variability of exchangeable potassium values in the field much greater than recorded for soils elsewhere; even for 3-ft spacings there was a sevenfold range. The need for adequate sampling of experimental areas is stressed. The median value for exchangeable potassium in the A₁ horizon of Frodsley sandy loam was 0.31 m-equiv./100 g for "developed" areas and 0.35 m-equiv./100 g for "undeveloped" areas. Values for the A₂ horizon were closely correlated with those for the corresponding surface horizon, but only about one-third as high. The amounts in the B horizon tended to approximate to those in the A₁ horizon except for soils with high values in the surface. The potassium-supplying capacity of the soils was assessed by fractionation of the potassium into water-soluble, exchangeable, difficultly exchangeable, hydrochloric acid-soluble, and total potassium. In Frodsley sandy loam approximately 35 per cent. of the exchangeable potassium appeared in the water-soluble form, which suggested that there may be moderate losses from this soil by leaching. Ten minutes' boiling with normal nitric acid released only about 90 per cent. more potassium than was present in the exchangeable fraction. Boiling normal nitric acid extracted much less potassium from some samples of this soil type than did cold normal ammonium chloride from others, because of the great variability of exchangeable potassium. Concentrated hydrochloric acid dissolved a further 0.60.7 m-equiv./100 g on the average. These low values for the two latter fractions are taken to indicate the poverty of this soil type in reserves of potassium and, with the relatively low values for the exchangeable fraction, explain the widespread responses to potassium reported in field experiments. Examination of the minerals of the clay fraction of Frodsley sandy loam supported the chemical data in regard to the poor potassium status of these soils. Separation of sand, silt, and clay from the B horizons of two profiles showed that much of the total potassium was present in the coarser fractions of the soil. Type A, a soil associated with Frodsley sandy loam on river terraces, had a similar potassium status. Soils formed on dolerite were significantly higher in all categories of potassium.